Various methods for the contact-free measurement of surfaces are known. In most of these methods, the light generated by the laser is focused in the desired position of the surface and the light reflected from this surface is conducted to a receiver which measures the intensity of the light. The light intensity is at its highest when the surface takes on its desired position. In order to establish a certain measuring region around the desired position, it is known to axially move a diaphragm periodically which is arranged in front of the receiver as disclosed in the article entitled "Photoelectric Detection of Displacement from Focus" in Applied Optics, Volume 5, Number 12, (1966), pages 1061 and 1062 or, the focusing range of the lens that images the laser light can be axially expanded as disclosed in "Feinwerktechnik & MeBtechnik 84", Book 2, (1976), pages 72 to 74. With these methods, a scanning as well as a profile measurement of the surface is possible; however, the achieved accuracy is insufficient for many applications.
Holography is another method for the contact-free measurement of the form of a surface and is disclosed in an article entitled "Testing Aspherics Using Two-Wavelength Holography" in Applied Optics, Volume 10, Number 9, (1971), pages 2113 to 2118. Here, an interferogram of the object to be examined is generated and retained photographically and placed in the measuring arrangement as a hologram. Measurements are then made with a light of a second wavelength and the object to be examined remains in the beam path. In this way, an interferogram is made that makes possible a very precise evaluation. However, this method has the disadvantage that work in real time cannot be carried out.
An attempt has already been made to measure rough surfaces interferometrically. For this, radiation having a wavelength of 10.6 .mu.m was utilized. In a Twyman-Green interferometer, interference strips are thereby generated having a contrast which becomes poorer with increasing surface roughness. In this connection, reference may be had to the article entitled "Rough Surface Interferometry at 10.6 .mu.m" appearing in Applied Optics, Volume 19, Number 11, (1980), pages 1863 to 1869. This method has the disadvantage that it functions with invisible radiation so that the adjustment of the interferometer is very complex and difficult.